In some applications, it is desirable to provide an optical displacement sensor which provides a linear displacement signal, in a manner analogous to that of LVDT-type sensors. One prior art system of this type is disclosed in U.S. Pat. No. 4,338,722 (the '722 patent), issued to Delmas, which is hereby incorporated by reference in its entirety. The '722 patent discloses a sensor with a light source, a detector comprising two photoreceiver detectors, and a moving stem with an opening to transmit light from the light source to the detectors. The moving stem slides between two guides. A cover over the opening on the moving stem is designed to be substantially symmetrical with the contact tip at the stem end relative to the center point between the two guides, as a means of eliminating measurement sensitivity to mechanical play of the stem. U.S. Pat. No. 4,812,635 (the '635 patent), issued to Kaufmann et al., which is hereby incorporated by reference in its entirety, provides a means for homogenous illumination in a position sensing device. The device of the '635 patent includes a light source, two photodiodes, and a moving diaphragm which contains an aperture. The two photodiodes produce a signal determined by the position of the light which passes to them from the light source through the aperture on the diaphragm. The means for homogenous illumination comprises a correction filter along the optical path, which in the preferred embodiment comprises a film negative exposed using the light source at its nominal spacing. Such a custom filter outputs illumination which nominally has a uniform intensity. However, the devices of the '722 and '635 patents suffer from remaining unrecognized errors associated with their light sources, which lead to signal nonlinearity and/or accuracy errors which limit their range to resolution ratio.
A high range to resolution ratio is a valuable performance characteristic of an analog optical position sensing device. U.S. Pat. No. 7,894,079 (the '079 patent) provides a range to resolution ratio which may be superior to that achievable by the devices of the '722 and '635 patents. The optical position sensing device of the '079 patent uses an advanced aperture configuration and a position sensitive detector for reliable, high resolution measurement. However, the device of the '079 patent also suffers from an undesirably limited range to resolution ratio. An optical position sensing device which provides an improved range to resolution ratio would be desirable.